1. Field of the Invention
This invention relates to a device for electronic focusing of ultrasonic waves.
2. Description of the Prior Art
Ultrasonic-wave devices are in particularly common use in the medical field and in industrial control. Their general function is to permit measurement of physical parameters of a medium in which these waves are transmitted and/or received. For the purpose of transmission, an ultrasonic device comprises an electrical signal transmitter connected to a piezoelectric excitation probe. For the purpose of reception, an ultrasonic device comprises a piezoelectric probe connected to a receiver. In conventional practice, a duplexer permits the use of a single reversible probe for transmission and reception while dividing the length of time between these two functions.
The mechanical pressure wave which propagates from a probe within a medium is a spherical wave. In consequence, the quantity of excitation energy applied at a particular point of the volume under study decreases with the square of the distance between said point and said probe. Moreover, the same phenomenon takes place at the time of reception. Thus the back-scattered signal originating from a particular point of the medium also propagates as a spherical wave. In order to increase the excitation power applied to a particular point of the medium at the moment of transmission, and in order to increase the energy of the signal received from a particular point at the moment of reception, it is known to provide probes comprising an array of piezoelectric cells. In order to permit achievement of expected effects, it is also necessary to establish time-delays between the electric signals which are applied to the different cells of the array. These time-delays are intended to focus the wave on the point considered and are obtained in accordance with conventional practice by employing delay lines connected in series with the cells. The time-delays of each delay line are fixed at values which are predetermined in respect of a given position of the point of the medium on which it is desired to focus the transmission wave or from which it is desired to receive the transmitted wave. Each cell is associated with a particular delay line and the number of delay lines corresponds to the number of cells.
Moreover, it is a desirable objective to have the possibility of employing ultrasonic devices for pointing them at different regions of the medium. These regions are contained between two extreme directions of relative angular displacement. It is advisable in this case to provide delay lines in which the time-delay is variable and in which the maximum time-delay corresponds to these extreme directions. Postulating, for example, a relative angular displacement of 45.degree. with respect to a normal axis of propagation at right angles to the array of cells, a propagation velocity of 1500 m per second within the medium and a distance of approximately 100 mm between the point sighted and the array, it is apparent that a wave which is again in phase at the point considered permits time-delay shifts between the different cells of the array. These shifts are such that, in particular, there exists a maximum differential time-delay of approximately ten microseconds between two end cells of an array which measures approximately 22 mm in length. In order to permit scanning of all points of the medium identically and symmetrically with respect to a line normal to the center of the array, the time-delays of the delay lines relating to the cells located at the ends of the array must be identical. However, the fabrication of delay lines is attended by many technical and technological problems. It is necessary to reduce the number of delay lines and especially the duration of their intrinsic time-delay.